1. Field of the Invention
This invention relates to flexible ultraviolet radiation transmitting filters, and more particularly to such flexible filters which transmit radiation in the ultraviolet region of the spectrum and absorb radiation in the visible region of the spectrum.
2. Description of the Prior Art
Dual response photosensitive compositions have been taught in U.S. Pat. Nos. 3,390,994; 3,390,996; 3,658,542; 3,658,543 and British Pat. No. 1,384,406. These compositions, which are photoimageable color-forming compositions, include such materials as hexaarylbiimidazoles and leuco dyes which form color on radiation with ultraviolet radiation, and can be deactivated against such color formation with radiation of a second wave length by incorporating therewith light-activatable oxidation-reduction systems, such as a visible light-activatable quinone in combination with a source of abstractable hydrogen. The manner in which the system operates is well documented. It is important to note, however, that hexaarylbiimidazoles which are preferred as photoactivatable oxidants absorb maximally in the region of 255 to 275 nm and usually show some lesser absorption in the region of 300 to 375 nm. The absorption bands tend to tail out to wavelengths as high as 420 nm. Thus radiation effective for imaging is in the range of 200 to 420 nm, preferably in the range of 255 to 375 nm. The deactivating component of the photoimageable color-forming compositions, which preferably is a polynuclear quinone, absorbs principally in the 400 to 550 nm region. Thus the radiation of the deactivating radiation source is in the range of 400 to 550 nm.
In practice, it would appear that many convenient sources of ultraviolet radiation can be used to activate the photoimageable color-forming compositions, whereas convenient sources of visible light can be used to achieve deactivation. Actually, however, few radiation sources have been found to be effective for color formation since the commonly-used radiation sources, e.g., those used in exposing diazo and photopolymerizable materials as well as xenon flash lamps, emit strongly in the visible region as well as the ultraviolet region. It has been found necessary to use optical filters to eliminate the visible components for effective color formation. The use of filters has been described in the aforementioned U.S. Patents.
As dual response photosensitive materials achieved greater commercial acceptance it was apparent that the filters described and available possessed disadvantages when used with standard radiation sources which emit radiation in a broad spectral range, e.g., 200 to 555 nm. One of the filters in common use is the Kokomo type 400, Kokomo Opalescent Glass Company. This glass type filter is useful with mercury vapor radiation sources. The Kokomo filter, however, has several disadvantages. It not only is breakable, but it is relatively expensive. In addition, an inventory of the glass should be maintained since it is not readily cut to any conceivable shape or size. The Kokomo filter has a tendency to break due to thermal absorption from the radiation source, thereby either requiring use of a cooling means or carefully positioning the filter some distance from the radiation source.